The U.S. Supreme Court got down and dirty Wednesday with a case about whether the Environmental Protection Agency's permit for San Francisco's aging sewer system is too vague, creating a potential land mine of billions of dollars of pollution liability.

The EPA has been at loggerheads with San Francisco in recent years over what the federal agency calls a "failing" sewer system, which, such as those of New York, Philadelphia and other older cities around the country, collects both rain and sewage into the same pipes.